The present invention relates to a method for controlling ignition timing of an internal combustion engine for an automobile through a learning operation.
A learning control system for correcting the ignition timing has been proposed. The control system is adapted to advance the ignition timing so as to produce a maximum torque as long as a level of an engine knock does not exceed a tolerable level. If the knock occurs, the control system operates to retard the ignition timing by a predetermined value. A maximum advance ignition timing for producing the maximum torque without causing knocking, depends on a type of the engine and on an octane number of fuel used in the engine. Namely, when high-octane gasoline or a mixture thereof with low-octane gasoline is used, the ignition timing is advanced from that of the low-octane gasoline for increasing the torque. Since the octane number varies with the quality of the fuel, the ignition timing varies accordingly so that it is necessary to correct the ignition timing through a learning control system.
The maximum advance ignition timing is advanced as the octane number of the fuel increases. However, it is not advisable to advance the timing only in accordance with the octane number, over a limit ignition timing for producing the maximum torque dependent on engine operating conditions because the torque is reduced.
Japanese Patent Application Laid-Open 61-157768 (U.S. Pat. No. 4,736,723) discloses a learning control system for the ignition timing control. In the system, a memory storing maximum advance ignition timings and lower limit ignition timings is provided, arranged in accordance with engine speed and intake passage pressure. The difference between a maximum advance ignition timing and a lower limit ignition timing derived from the memory is calculated. A correcting value is obtained by multiplying the difference by a coefficient as a dividing point of the differential period between the maximum advance ignition timing and the lower limit ignition timing. The ignition timing is obtained by adding the correcting value to the lower limit ignition timing. However, the calculated ignition timing does not coincide with a theoretically desired maximum timing. For example, in a low engine speed range the timing is advanced too much from the desired maximum timing, and in a high engine speed range the timing is largely retarded. Consequently, the ignition timing is not properly controlled so as to produce a maximum torque by selected fuel.
In addition, since the ignition timing is determined at the dividing point of the differential period between the maximum advance timing and the lower limit ignition timing, the ignition timing can not be advanced over a limit.